This invention relates to semiconductor devices including a periodic structure known as a superlattice and more particularly to such a device which is adapted to operate as a detector and/or mixer of incident microwave and infrared radiation.
Semiconductor devices having a superlattice structure in which there is a periodic variation in semiconductor composition is known having been shown and described, for example, in U.S. Pat. Nos. 3,626,257 and 3,626,328 issuing to Leo Esaki, et al. on Dec. 7, 1971. The teachings of these patents are herein incorporated by reference. The layered superlattice structure is formed by either doping or alloying techniques. In doping the superlattice structure is reduced by epitaxially growing a semiconductor material which is periodically doped so as to produce alternating multi-thin layers having different conductivity types. The alternate technique of alloying involves growing ultra-thin layers with varying alloy compositions. Suitable combinations of semiconductor material for the construction of the thin periodic layers include, for example, GaAs-AlAs, GaSb-AlSb, InAs-GaSb, GaAs-ZnSe, GaSb-ZnTe, InSb-CdTe and their respective alloys.
Sensitive and fast semiconductor detectors and/or mixers in the microwave and infrared (IR) regions of the electromagnetic spectrum are presently of great technological importance. In these devices various physical phenomena are exploited so that the incident radiation modulates some easily measurable parameter of the detector. Most common are the detectors where the conductivity of the device is monitored and in all cases, the radiation results in an increase in the conductivity due to carrier heating. Typically these devices contain shallow traps and have to be cooled in order to freeze out the carriers. The radiation excites the carriers out of these traps, thus creating a surge in conductivity. To date a great deal of effort has been expended to understand the scientific aspects and to perfect the technology of these devices; however, the problems in efficient microwave and IR detection remain numerous. The cooling requirements (often helium temperature operation is necessary) are cumbersome; the response time is unsatisfactory because carriers have to be retrapped which is a typically slow process; and the bandwidth of most of these devices is limited.
This invention is directed to a detector/mixer based on an entirely different effect which results in overcoming the above mentioned fundamental limitations.
It is an object of the present invention, therefore, to provide a superlattice structure which is operable over a relatively wide frequency band.
It is another object of the present invention to provide an improved superlattice structure which is responsive to microwave and infrared radiation.
Yet another object of the present invention is to provide a superlattice structure which is operable at room temperature.
Still another object of the present invention is to provide a superlattice structure which does not have a requirement that the device be cooled for proper operation.
Still another object of the present invention is to provide a superlattice structure which is relatively conductive in the absence of radiation, but becomes relatively less conductive in the presence of radiation.